The present invention relates generally to a satellite communications system, and more particularly it relates to compensation for the frequency drifts of transponder frequency translator.
In satellite communications network, all the frequencies of downlink channels vary simultaneously due to the frequency variation of a local oscillator of the satellite transponder in a range between 10 kHz and 100 kHz, whereas the frequency variation range of the earth station is usually less than 5 kHz. With single channel per carrier (SCPC) transmission having a separation of 25 kHz between adjacent channels, for example, the transponder frequency drift will cause an earth station to receive signals on a wrong channel which may be spaced more than one channel away from the intended channel. To compensate for the transponder frequency drifts, a pilot signal is currently transmitted on each down-link and the earth station uses it as a reference frequency to detect the amount of frequency deviation. However, the current practice adds to the total cost of the earth station. One solution to this problem would be to transmit a channel identification code identifying each of the uplink and downlink channels of a SCPC system. Since there is a substantial number of satellite links in such a system, the channel identification code will be represented by a long string of bits, which would lessen the transmission efficiency of the system.